There are many applications which require the use of filters whose frequency response characteristics are variable. One such application is that of vibration monitoring. In such vibration monitoring systems, one or more transducers are employed to sense vibration levels and output an AC signal having a frequency content and amplitude which are characteristic of the sensed vibration levels. The output of the vibration transducer is generally input to a filter network which sharply attenuates those signals not within a particular band of frequencies which are characteristic of the vibration signals of interest. When required to measure vibration levels within several different spaced frequency bands, prior art vibration monitoring systems have utilized a variety of techniques. Such techniques have included the use of separate filters, each having a desired response characteristic. Alternatively such prior art vibration monitoring systems have used a single filtering network whose frequency response characteristics may be selectively controlled either manually or automatically. Manually variable characteristic filters are undesirable because of the time required to adjust them. Prior art automatically adjustable filters have generally failed to provide the required accuracy at all frequencies. One such prior art system uses tracking filters which employ super heterodyne techniques. However, super heterodyne tracking filters have exhibited long term instability and lack the accuracy levels generally required for many vibration monitoring applications. Because the super heterodyne tracking filter and other automatically variable characteristic filters of the prior art have generally lacked the accuracy and long term stability required for vibration monitoring techniques, the prior art vibration monitoring systems have generally used separate filtering networks for each filtering characteristic desired or time consuming manually adjustable filtering networks.
It is therefore a primary object of this invention to provide an electronic filter which exhibits high accuracy and long term stability characteristics and which has frequency response characteristics that may be automatically controlled as a function of the magnitude of an applied DC control voltage.
It is a further object of this invention to provide an electronic filter, the center frequency and/or Q of which may be selectively controlled by the magnitude of an applied DC voltage.
It is still a further object of this invention to provide a system for monitoring vibration levels in a gas turbine engine which incorporates the voltage controlled filter of this invention and wherein the center frequency of the filter is automatically varied as a function of gas turbine rotational velocity.
It is a further object of this invention to provide a voltage controlled electronic filter for use in a vibration monitoring system which includes an adjustable feedback circuit which may be set to reduce variations in phase shift as the filter center frequency is varied or alternatively to automatically track a predetermined frequency.